1. Field of the Invention
The present invention relates to a casting method of using a granularly crystallized magnesium alloy solid as a casting material, melting the solid material to state in which a solid-phase and a liquid-phase coexists to form metal products by a pressure casting means, and to their metal products.
2. Description of the Related Art
In a conventional manufacturing method for a product from a partially molten metal, a molten alloy is held in a state in which a solid-phase and a liquid-phase coexist in an insulating vessel for a desired time, generating many fine spherical primary crystals and pressure casting them in a mold of a die cast machine at a desired liquid phase ratio to form cast products (see for example Patent Reference 1).
Further, in molding of metal products using a magnesium alloy, a solid material potentially holding thixotropy is heated in a partially molten state and the obtained material is charged into a mold by an injection apparatus (see for example Patent Reference 2)
[Patent Reference 1 ]
Japanese A-HEI 9-10893 (on pages 3 to 5, FIG. 9)
[Patent Reference 2]
JP-A-2001-252759 (on pages 6 and 7, FIG. 1)
The above-mentioned patent reference 1 describes a pressure casting method of cast products by a die cast machine including the steps of pouring a molten alloy into a tilt cooling jig held at a temperature lower than the melting point to flow down and holding the alloy in an insulating vessel in a state at a temperature equal to or lower than a liquidus line temperature and higher than a eutectic temperature or a solidus line temperature for five seconds to 60 minutes to form the cast products in a liquid phase ratio of 20 to 90%, preferably 30 to 70%.
Further, the Patent Reference 2 describes a casting method including the steps of allowing a molten magnesium alloy to flow on a cooling tilt plate to cool until partially molten, reserving the alloy in a reservoir until it becomes a metal slurry having fine spherical crystals, then solidifying the slurry by rapidly cooling it to form a metal material potentially holding a thixotropic performance, and melting the metal material into a partially molten state magnesium alloy exhibiting a thixotropic performance for charging into a mold by an injection apparatus.
In the prior art described in Patent Reference 1, after a molten alloy is cooled to a partially molten state the obtained alloy must be held in an insulating vessel until it has a desired liquid phase ratio. Thus it takes much time from the melting of the material to the pressure casting of products. In order to shorten the time, many insulating vessels and their transfer means are required. Further, since the material is cooled close to a lower temperature that is close to the casting temperature and is transferred to a molding machine to conduct molding immediately, some molding machines have a problem, and cannot be adapted.
Even in the prior art described in Patent Reference 2, since the solid phase ratio of a partially molten slurry is high, so much time is also needed for potentially holding the thixotropic performance. However, the molding steps of remelting a rapidly cooled solidified metal material in a partially molten state by a molding machine and pressure casting the obtained material into a mold in a state having thixotropic properties can be completed in a short time. Further, the supply of the metal material into the molding machine is also easy, and continuous casting is also possible, whereby the prior art is adaptable to the casting machine.
However, in Patent Reference 2, it is difficult to set temperature conditions and a holding time for metal slurry crystals that are crystallized out at a solid-phase and liquid-phase coexisting temperature region to be uniform spherical crystals, and there is a problem in maintaining a solid-phase ratio, which is preferable for molding. The present inventors have studied these problems. As a result they have found that even if crystals do not become uniform spherical ones, if a primary crystal ratio in which a solid material is granularly crystallized is within a certain range, the primary crystal becomes a sphered solid phase in a solid-phase and a liquid-phase coexisting state, and at the same time a main solid phase has a solid-phase ratio preferable for casting in a grain size of 50 μm or more. Also, if the holding time is within 30 minutes, the material is pressure cast into molds without changing the set conditions so that a number of metal products of a magnesium alloy that are extremely excellent in a distribution state of primary crystals can be formed.